In previous experiments, three fragments of type IV collagen (called F1, F2 and F3) were isolated from the adult chicken gizzard and characterized (Mayne and Zettergren 1980). From analysis of the fragments, it was proposed that the native conformation of type IV collagen is a single molecule written [Alpha(IV)]2Alpha2(IV). Recent experiments measuring the lengths of type IV collagen fractions observed in the electron microscope after rotary shadowing have led to a model in which four type IV molecules are crosslinked at a domain called 7S. From 7S the order of the fragments of each molecule is considered to be 7S-F3-[F1]2F2. In order to confirm the model, future experiments will involve the isolation and characterization of type IV collagen fragments obtained after pepsin or trypsin digestion. Trypsin and pepsin cause different cleavages in the type IV collagen molecule, trypsin apparently cleaving preferentially between 7S and F3, and pepsin between F3 and [F1]2F2. Cyanogen bromide peptides will be prepared from F3, F1 and F2 and characterized. Together with observations of type IV collagen digests in the electron microscope after rotary shadowing, the order of the three fragments will be established. Lathyritic gizzards will be used for most experiments, although similar results have been obtained with the chicken kidney. Conventional antibodies have already been prepared against chicken type IV collagen, and will be used to investigate basement membrane formation during the development of skeletal muscle, heart and the major arteries. Monoclonal antibodies have also been prepared against native type IV collagen, and an attempt will be made to localize each determinant to the 7S, F3 and [F1]2F2 domains. If successful, the monoclonal antibodies will be used as tools to investigate type IV collagen structure. Organ cultures of chick lens capsule will be used to investigate the biosynthesis of intact type IV collagen molecules, and experiments with this tissue will complement the structural analyses. Changes in basement membrane structure occur in several diseases including diabetes, and the ability to degrade type IV collagen has been correlated with tumor metastasis. Analysis of the fragments of chicken type IV collagen obtained after digestion of proteases appears to offer a unique opportunity to investigate the structure of this collagen.